I. Field of the Invention
This invention relates generally to circuits capable of producing an output signal which adopts one of two discrete levels in response to differences in magnitude between a pair of analog input signals and more specifically to such circuits which possess input signal hysteresis characteristics.
II. Description of the Prior Art
Comparator circuits are used to detect whether a first input signal is above or below a second input signal by a preselected amount and indicate this relationship by means of an output signal or signals. Those comparator circuits possessing a single output terminal on which one of two selected output signals appear, depending on the relationship between the two input signals, are said to have a single-ended output. Such an arrangement contrasts with comparator circuits having differential outputs. In these circuits, one input signal relationship is indicated by a high-level signal on a first output terminal and a low-level signal on a second output terminal. The other input signal relationship is indicated by the high- and low-level output signals appearing on opposite output terminals. Comparator circuits with single-ended outputs are frequently easier for logic systems designers to use and therefore are more desirable than comparators with differential outputs.
Noise in the input signals to a comparator circuit can produce incorrect output indications if the input differential is near the threshold level which divides one ouput indication from the other. The problems that input signal noise create are reduced by inroducing hysteresis into the relationship between input signal differentials and ouput signals. This is accomplished by separating the threshold points at which the output indication changes such that increased absolute values of input signal differentials are required to achieve a change in the output signal.
The gain of a comparator circuit, as referred to herein, is the relationship of the input voltage change required for a given ouput voltage change. Although comparator circuits are known which have both high-current and high-voltage gain, previously described integrated comparators have achieved the hysteresis function at the expense of additional circuitry and a reduction in voltage gain. In addition, known comparator circuits employing multiple cascaded circuit stages are frequently unsatisfactory for many applications requiring high accuracy; (i.e., sharp edges on the output waveforms) where the analog input signals vary slowly in magnitude with respect to each other over time. Moreover, because most present day comparator circuits are embodied as integrated circuits in chips of semiconductor material, complex comparator circuits having many individual circuit elements are generally less attractive than simpler circuits. This is because complex circuits generally require more chip area than simpler circuits (a factor which inversely affects the process yield), and therefore cost more to fabricate.
Two examples of typical prior art comparator circuits are diclosed in U.S. Pat. No. 3,628,059 entitled "HIGH-VOLTAGE FUNCTIONAL COMPARATOR" issued to George G. Y. Niu on Dec. 14, 1971 and U.S. Pat. No. 3,848,139 entitled "HIGH-GAIN COMPARATOR CIRCUIT" issued to James G. Holt, Jr., on Nov. 12, 1974. Both of these patents are assigned to the assignee of this application.